1. Field of the Invention
The disclosed invention relates to efficient power amplifiers and more particularly to efficient RF power amplifiers.
2. Description of the Related Art
Radio Frequency (RF) transmitters, such as cellular telephones, use an RF Power Amplifier (PA) to provide the RF signal strength needed for radio communications over a distance. The output of the PA is typically provided to a transmitting antenna, and thus the power output of the PA is proportional to the transmitted power. As the output power of the PA increases, the power radiated by the transmitting antenna increases and the useable range of the transmitter increases.
In most RF transmitters, the PA handles the largest power within the transmitter and inefficiency in the PA typically accounts for much of the wasted power in the transmitter. Unfortunately, in many applications, the PA does not perform the task of power amplification efficiently, consuming much more power than is actually transmitted. This excess power generation can be costly, especially in battery operated devices, because it often necessitates the use of larger-capacity batteries, and/or shorter battery recharging intervals.
A PA may be designed to amplify an RF signal with a constant envelope or an RF signal with a non-constant envelope. A PA designed for constant envelope signals is typically more efficient than a PA designed for a non-constant envelope signal because the biasing circuits in the constant envelope PA can be optimized to deliver the constant power level. Moreover, if perfect envelope magnitude fidelity is not required, the PA circuits can be driven slightly into compression (nonlinearity), which offers even further efficiency gains. Unfortunately, as the PA is driven into compression, the signal spectrum tends to widen, due to nonlinear distortions. These nonlinear distortions produce intermodulation products, which arise when signals of differing frequencies pass through a nonlinearity. This spectral widening is called spectral regrowth, and is undesirable because it spills RF energy into adjacent frequency channels. The energy spilled into other channels is known as Adjacent Channel Power (ACP) emissions and is often undesirable because it may cause interference with communication systems operating in the other channels. Thus, tradeoffs exist between efficiency and ACP emissions, even for constant-envelope PAs. Typically, regulations on RF transmitters and communication systems specify an acceptable Adjacent Channel Power Ratio (ACPR). The ACPR is the ratio of the average power in the active channel passband to the average power spilled into an adjacent channel passband or some fraction of the adjacent channel passband.
Non-constant envelope signals, such as xcfx80/4 DQPSK (Differential Quadrature Phase Shift Keying) and spread spectrum signals, make the PA efficiency problem even more difficult because the modulation may cause the amplitude of the envelope to vary by 14 dB or more. Moreover, the peak-to-average power values may run from 3 dB, as in xcfx80/4 DQPSK, to 17 dB for some CDMA systems. Peak-to-average power is important because clipping occurs when the peak-power capabilities of the PA are exceeded, and clipping introduces much distortion. Most systems are biased so that the PA runs at near saturation when the amplitude of the envelope is at a maximum, corresponding to peak power output. To become even more efficient, some systems push the peak power output into saturation, but this can result in unacceptable ACP emissions. This push into saturation can also cause distortions in the in-band modulation accuracy, which is called EVM (error-vector modulation) accuracy, and is specified in terms of RMS error from an ideally modulated signal. To realize good ACP and EVM figures, transmitters that drive the PA into saturation often pre-distort those sections of the input envelope which would be compressed during saturation, so that the resulting output is an undistorted facsimile of the input. Unfortunately, most saturation regions are narrow (often less than the 3 dB peak-to-average criterion given above for xcfx80/4 DQPSK), and thus only allow modest pre-distortion (and efficiency) improvements.
Prior art PA designs are particularly inefficient when operating at less than full output power, as is common in systems that use adaptive power control. With adaptive power control the system controls the output power of the PA such that the PA provides only as much output power as is needed to provide good communications. Adaptive power control is useful because it extends battery life, by transmitting with no more power than needed, and, at the same time, increases a communication system""s capacity, by reducing the interference among users. However, many of the desired gains promised by adaptive power control have not been realized because the power saved by transmitting at reduced power is lost because the PA is less efficient at reduced power. In one prior implementation, a typical prior art PHS (Personal Handy phone System) transmitter is typically 25% efficient at full power but only 3% efficient at a nominal power reduction of 10 dB. Similar performances have been observed for other modulation schemes and communication systems.
The present invention solves these and other problems by providing an envelope feedforward PA design that improves the efficiency of a PA. Efficient operation is provided for signals with a non-constant, as well as constant, envelope. Efficient operation is also provided when operating the PA at reduced power levels. The PA may advantageously be used with many wireless communications systems because power control and high efficiency are universally desired features in subscriber units, as well as base stations. Even though battery life is not generally a problem in a base station, base stations benefit from a more efficient PA because the PA can be made relatively smaller, and the PA can be connected to circuits with lower power delivery ratings.
Virtually all communication systems employing a non-constant envelope modulation scheme, including, for example Personal Handy System (PHS) telephones, CDMA and spread spectrum telephones such as the Rockwell Spread Spectrum Telephone (SST), IS-95 [Electronic Industries Association/Telecommunications Industry Association; 1993] telephones, IS-136 telephones, and Personal Digital Cellular (PDC) telephones will benefit from the envelope feedforward PA. In one embodiment, the envelope feedforward PA is advantageously applied to PHS transmitters to increase PA efficiency to 70% or more.
Even communications standards employing constant envelope modulation schemes, such as Advanced Mobile Phone Service (AMPS), Global System for Mobile Communications (GSM), and Digital European Cordless Telephone (DECT) systems benefit from the power control feature provided by the envelope feedforward PA. The envelope feedforward transmitter is also useful with wideband systems such as CDMA and spread spectrum systems (e.g., IS-95, etc.).
In another embodiment, the feedforward transmitter provides higher efficiency over a range of nominal power output settings. Higher efficiency over a range of power output settings is desirable to support adaptive power control.
In one embodiment, an input signal having phase and amplitude components is separated (decomposed) into a phase and frequency modulated (FM) portion and an envelope portion. The FM portion has a constant envelope (no amplitude modulation) and contains the phase and frequency information from the input signal. The envelope portion contains the amplitude information from the input signal. In a multistage amplifier, the FM portion is amplified by a first amplifier to produce an amplified FM signal, and the envelope portion is optionally amplified by a second amplifier. In some embodiments, xe2x80x9camplificationxe2x80x9d of the envelope portion may be accomplished by adjusting the reference voltage in a digital-to-analog converter. The envelope signal is combined with the amplified FM signal by an envelope combiner to produce a combined signal having FM and amplitude components. The envelope combiner uses the envelope signal to amplitude-modulate the FM signal to produce the combined signal.
The feedforward PA also provides precise and efficient power control to allow the transmitter to be operated at reduced power levels without sacrificing efficiency. Over a certain range of power outputs, adjusting the nominal (average) power at the PA output is accomplished by level shifting the envelope component. One method of accomplishing this is by intelligently changing the output range of the DAC (Digital-to-Analog Converter) responsible for the envelope component at baseband. Alternatively, a bias voltage associated with the envelope control input is adjusted. One skilled in the art will recognize that a combination of the above techniques may be used.
In one embodiment, the envelope combiner uses a gain-controlled amplifier having a signal input and a control input. The gain of the gain-controlled amplifier is a function of a signal provided to the control input. The amplified FM signal is provided to the signal input and the amplified envelope signal is provided to the control input. An output of the gain-controlled amplifier is the combined signal.
In another embodiment, the envelope combiner uses a mixer with conversion gain to combine the amplified FM signal and the amplified envelope signal. The amplified FM signal is provided to one input of the mixer and the amplified envelope signal is provided to another input of the mixer. An output of the mixer is provided to a filter, and an output of the filter is a combined signal.
In yet another embodiment, the envelope combiner uses a voltage-controlled switch in combination with a variable gain element to combine the FM signal and the envelope signal. The FM signal is provided to a control input of the switch. The amplified envelope signal is used to alter the transconductance of a variable gain element, such as a FET operating in its linear (saturation) region. The envelope is superimposed on the FM signal by altering the gain of the variable gain element. In one embodiment, the combination of a voltage-controlled switch and a variable gain element is provided by a dual gate FET.
In one embodiment, the relative time delays in the signal path of the envelope signal and the FM signal are matched such that the amplified non-constant signal and the amplified FM signal arrive at the envelope combiner at the same time. The delay in the envelope signal path is tuned to the delay in the FM signal path. Proper adjustment of the relative time delay in the two paths reduces spectral regrowth associated with the constant envelope signal input. Real-time adjustment of the delay (tuning) is typically not needed. However, real-time adjustment of the delay may be advantageously provided for transmitters that are not sufficiently stable over time and temperature. Temperature sensors and a control algorithm are used to adjust the delay timing.
In one embodiment, an input signal having FM and envelope components is produced by an encoder that produces a modulated signal represented by an I channel output and Q a channel output. The modulated signal is separated (decomposed) into an FM portion and an envelope portion by using a lookup table having cells arranged as rows and columns. A cell is selected (addressed) by using I channel and Q channel values ad row and column addresses. Each cell in the lookup table provides data corresponding to the envelope signal, the I portion of the FM signal (IC), and the Q portion of the FM signal (QC). In one embodiment, the IC and QC portions are combined by a quadrature mixer to produce the FM signal.
In one embodiment, an input signal is separated into a first signal and a second signal, the second signal having an envelope amplitude maximum and an envelope amplitude minimum. The first signal is modified to produce a first modified signal, the first modified signal being delayed by a first propagation delay. The second signal is modified to produce a second modified signal, the second modified signal being delayed by a second propagation delay. The first modified signal and said second modified signal are combined to produce an output signal. The lesser of the first propagation delay and the second propagation delay are adjusted to reduce spectral regrowth of the output signal. In one embodiment, an absolute value of a difference between the first propagation delay and the second propagation delay is less than one microsecond. In one embodiment, an absolute value of a difference between the first propagation delay and the second propagation delay is less than one tenth of a time bandwidth of the input signal. In one embodiment, the propagation delay adjustment includes increasing said lesser of said first propagation delay and said second propagation delay.
In one embodiment,. the envelope minimum is not less than 75% of the envelope maximum